1. Field of the Invention
The present invention relates to a projection lens for use in e.g. a projector having a zoom function, and more particularly to a projection lens for enlarging and projecting an image generated by e.g. a liquid crystal panel or a digital micromirror device onto a screen.
2. Description of the Related Art
There is widely known use of a glass material having an anomalous dispersion characteristic for correcting lateral chromatic aberration of a lens. Unlike a general glass material, the glass material having an anomalous dispersion characteristic has a feature that a graph representing a relation between a partial dispersion ratio and the Abbe number is greatly displaced from a straight line.
The partial dispersion ratio Pg, F; and the Abbe number Vd are respectively represented by the following conditional expressions.Pg,F=(ng−nF)/(nF−nC)where ng is a refractive index with respect to g-line, nF is a refractive index with respect to F-line, and nC is a refractive index with respect to C-line.Vd=(nd−1)/(nF−nC)where nd is a refractive index with respect to d-line.
In the specification, as compared with a dispersion characteristic of a general glass material, a lens element made of a glass material having relatively large refractive indexes with respect to red light and blue light is called as a lens element made of a glass material having an anomalous dispersion characteristic of Lang (hereinafter, simply called as “a lens element having Lang characteristic”); and a lens element made of a glass material having relatively small refractive indexes with respect to red light and blue light is called as a lens element made of a glass material having an anomalous dispersion characteristic of Kurz (hereinafter, simply called as “a lens element having Kurz characteristic”).
Specifically, a lens element made of an anomalous dispersion glass material has the following features.
i) In the case where a positive lens element having Lang characteristic is disposed on the reduction side of a diaphragm, lateral chromatic aberration occurs on the minus side.
ii) In the case where a positive lens element having Lang characteristic is disposed on the enlargement side of a diaphragm, lateral chromatic aberration occurs on the plus side.
iii) In the case where a negative lens element having Lang characteristic is disposed on the reduction side of a diaphragm, lateral chromatic aberration occurs on the plus side.
iv) In the case where a negative lens element having Lang characteristic is disposed on the enlargement side of a diaphragm, lateral chromatic aberration occurs on the minus side.
In the specification, the expression “lateral chromatic aberration occurs on the plus side” means that a projected image of red light or blue light is formed at a position closer to the center of a screen with respect to a projected image of green light; and the expression “lateral chromatic aberration occurs on the minus side” means that a projected image of red light or blue light is formed at a position closer to a periphery of a screen with respect to a projected image of green light. In the case where a lens element having Kurz characteristic is used, lateral chromatic aberration occurs in a reverse manner as described above.
In recent years, high magnification zoom lenses compatible with various photographing conditions have been demanded. In the case where the zoom ratio is increased in the high magnification zoom lenses, it is difficult to obtain an optimal lens arrangement that enables to minimize lateral chromatic aberration at both of the wide-angle end and the telephoto end. In order to satisfy the above requirement, an increased number of lens elements is required, and the production cost may be increased.
In the case where the zoom ratio is large, the positive power by a positive lens element is dominant at the wide-angle end where the power is increased among all the lens groups. In view of this, there is proposed an idea, wherein an optimal lens arrangement is set at the telephoto end, and a movable lens group is disposed in front of a diaphragm to suppress variation in telecentricity. In this arrangement, however, correction by the positive lens element may be excessive, and lateral chromatic aberration may occur on the plus side.
On the other hand, lateral chromatic aberration may occur on the minus side at the telephoto end. In other words, as a principle, lateral chromatic aberration is likely to occur on the plus side at the wide-angle end, and lateral chromatic aberration is likely to occur on the minus side at the telephoto end.
In order to correct the lateral chromatic aberration, U.S. Pat. No. 5,278,698, JP2004-109896, U.S. Pat. No. 7,170,691, and JP2005-43607 disclose zoom lenses, wherein a lens element having a large anomalous dispersion characteristic is arranged in a lens group having a large moving amount.
In the zoom lens recited in U.S. Pat. No. 5,278,698, since the moving amount of a lens group on the reduction side is large, telecentricity may vary by a zooming operation.
In the zoom lenses recited in JP2004-109896, U.S. Pat. No. 7,170,691, and JP2005-43607, the distance between a lens group having a maximum moving amount, and a diaphragm is not significantly changed during a zooming operation. Accordingly, the light flux passing position through the lens group is not significantly changed during the zoom operation, and lateral chromatic aberration which may vary by a zooming operation cannot be sufficiently corrected. In other words, in the zoom lenses recited in JP2004-109896, U.S. Pat. No. 7,170,691, and JP2005-43607, the effect of correcting lateral chromatic aberration is substantially the same between the wide-angle end and the telephoto end. Accordingly, a difference in lateral chromatic aberration between the wide-angle end and the telephoto end may remain unchanged, and fine lateral chromatic aberration correction cannot be performed in the entire zoom range.